Sejuani is a legitimately weak champion, she really doesn't do much of anything and her only saving grace is her ult. She doesn't do much damage, she can't really peel, she can't soak damage, she only has one hard CC, her slows are really mediocre without E...

Tanks are really... ****ed up in LoL. There's debate how they should soak damage; taunt is one-trick, doing too much damage that requires focus is just a bruiser (and arguably OP), providing CC is one-trick where the enemy team no longer focuses you when your CC is on cooldown, providing utility for your team (ala Leona) doesn't attract much attention and you're team-dependent.
I don't think the act of tanking is difficult, it's the act of getting people to focus you so you can tank that is difficult.

I think another argument I've been hearing is that the difference between a tank and any other champion is very little because resistances aren't that effective, really.

Since when are tanks supposed to deal damage? They're supposed to soak it, not deal it.

Dot kinda touched on it, but if you can't deal any damage, you're not going to have to soak any damage because you're not the prime target.

Amumu is a meh tank because he's basically a grenade. QR and then he's basically useless. Malphite's a meh tank because (imo) he does too much, he's more of a bruiser than anything. Sejuani and Leona are bad because they deal no damage. The other tanks fall into some spectrum of the other four I just mentioned.

Dot kinda touched on it, but if you can't deal any damage, you're not going to have to soak any damage because you're not the prime target.

Amumu is a meh tank because he's basically a grenade. QR and then he's basically useless. Malphite's a meh tank because (imo) he does too much, he's more of a bruiser than anything. Sejuani and Leona are bad because they deal no damage. The other tanks fall into some spectrum of the other four I just mentioned.

Amumu is banned in ranked games pretty much 100% of the time, if people are so scared to even play against him then I'd say he's doing a pretty good job at "being a threat" like you said, except not necessarily by damage.

Leona DOES NOT deal no damage, if anything, she deals too much damage, seriously do you expect a character with ~5 secs of hard CC AND a 70 armor+MR steroid to also be dealing insane AoE damage too (reminder, she has 3 AoE abilities and a spammable single target)? Her base damage and her passive hurt A LOT already.

Leona doesn't do damage herself. Her base may be good, as expected of a tank, but her passive is purely team-dependent. By mid-late game, the damage she deals isn't enough to focus her, the CC she does only works until she uses all her cooldowns. The ONLY reason anyone focuses tanks is because they're easy to kill now.

Actually, don't care too much for Coronach's champions. He's made Lee Sin, Poppy, Mordekaiser, and Olaf, as well as the remake for Kayle (which was a flop). I absolutely abhor those champions, especially Lee Sin.